A typical boiler, used for heating purposes, consists in a hot water container which has thermal insulated walls, an outlet nozzle for water consumption and an inlet nozzle for water replacement. In this system, the water temperature is usually increased and maintained with a desired range by on electric or a gas heater. For saving energy during water heating, the basic system may be combined with a solar power supply.
In energy storage systems, it is desired an increase of the heat storage capacity and a reduction of the heat losses. A known way to increase the storage capacity is by using phase change of a material with a high latent heat of fusion. For a boiler, this should be done by introducing into a closed reservoir a melting material, such as paraffin, which has its melting point between the operational temperature range of the boiler. Therefore this boiler will store more energy than a conventional one, without paraffin.
In a system, as described above, the energy accumulation process is the following: during heating, the energy is stored by sensible heat of the water and the paraffin. When the melting point is reached, the heat absorption rises due to the phase change and it begins to store a great amount of energy by latent heat of fusion without any temperature increase. In conventional boilers, with the same volume of water, it will be necessary a great temperature increase to store the same energy value.
While hot water is replaced by cold water during the use, the temperature of the resulting water mixture and the paraffin reservoir drop. When the paraffin reaches its solidification temperature, it starts to solidify and discharge a great amount of heat to the water, without changing the temperature, as in the melting process. Together with the increase of energy storage capacity, the introduction of paraffin minimizes the heat losses to the environment, for the same amount of energy stored. Since water temperature is smaller in a boiler with paraffin reservoir and sure the heat losses are proportional to the temperature difference between boiler and the surrounding, the system wastes less heat.
To be useful, this new system must maintain enough hot water volume to guarantee the supply even during peaks of use and the time necessary to have the heat transfer from the paraffin to the water. In practice, the system will not present any advantage if, even storing a great amount of heat, it cannot replace quickly heat of the consumed water. Then, the boiler efficiency is measured by the speed with which the paraffin heats the water after it is cooled by the make up water or by the waste of heat to the surrounding.
The paraffin reservoir must provide an interface between water and paraffin, made of a high thermal conductive material in order to allow a fast heat transfer condition, either from water to the paraffin during the heating, or in the opposite direction when the water is colder than the paraffin.
Nevertheless, there is a problem during the heat transfer from paraffin to water. The solidification process starts near the interface wall and a solidification front spreads into the reservoir. As the solid paraffin near the wall acts like a thermal insulation, it results in a temperature stratification, slowing the system performance. The goal of this invention is to minimize the stratification effects and to prevent the insulation layer of paraffin from solidifying.